With the increased use of powdered polymeric coatings or films, interest has grown in fusion bond coating processes for inexpensively protecting metal surfaces for a variety of applications. These include protection from outdoor weathering, low order corrosion (such as encountered in subsurface burial applications) high order corrosion, and for extending the "life" of electrical insulation on rotors, stators, coils, and assorted pole line hardware. While there are several different powder coating techniques available, the instant invention is directed toward resin powders for electrostatic spray coating processes.
Electrostatic spray coating processes are defined herein refers to procedures that operate on the principle that oppositely charged particles attract each other. Resin powders are fed from a reservoir, often from a fluidized bed to a "gun" by air pressure. There a high voltage, low amperage electrostatic charge is applied. The resin powder picks up the charge due to a transfer of electrons from gun to powder. This transfer takes place both through contact with the highly charged gun nozzle and through the surrounding ionized air.
Interest in electrostatic processes has substantially increased in view of the development of powdered resins with powdered cross-linking agents along with auxiliary agents promote the application of plastic coatings without the necessity of employing solvents thereby minimizing air pollution.
Two-component systems of hydroxy-containing copolymers and polyisocyanate as the cross-linking agent form a special group of useful resin powders. The polyisocyanate cross-linking agent is employed in blocked form in these systems, i.e., all isocyanate groups are blocked with agents that can be split off only at baking temperatures so that the mixture of hydroxyl groups-containing copolymer and cross-linking agent has good storage properties at ambient or slightly elevated temperatures.
For example, German Auslegeschrift No. 1,957,483 discloses a powdered coating material of polyester containing hydroxyl-groups and of a .epsilon.-caprolactam-blocked polyisocyanate, e.g. hexamethylene diisocyanate. This system does not react when being short time heated to temperatures of up to 130.degree.C., and is curable at temperatures of about 150.degree.C. The Auslegeschrift also mentioned that partially blocked isocyanates, e.g. mono-.epsilon.-caprolactam-blocked diphenylmethyl-4,4'-diisocyanate, can also be employed but it is stated that no more than one free isocyanate group may be present per molecule otherwise a premature cross-linking of the components may take place during the mixing of the two components.
Another relevant disclosure German Offenlegungsschrift No. 2,105,777 describes a two-component coating resin mixture. One component of the mixture is a polyester containing hydroxyl groups, the second component is .epsilon.-caprolactam-blocked isophorone diisocyanate which is used as the cross-linking agent. Two-component resins with volatile blocking agents are in some respects disadvantageous: A sizable portion of the blocking agent is lost as a gas during the baking of the resin mixture. This results not only in a loss in material but also in a bad effect on the surface appearance of the spray coatings. The greater the loss in blocking agent, the more is the film quality deteriorated.
In the process disclosed in German Auslegeschrift No. 1,957,483 either a single-component or two-component coating resin or lacquer can be produced. However, the use of an aromatic diisocyanate as the cross-linking agent as claimed in said Auslegeschrift impairs the coatings' resistance to light and weather.
As can be appreciated from the discussion supra the development of a single-component coating resin or lacquer which would obviate the problems and disadvantages of the prior art two-component resin mixtures would represent a substantial advance in the art.
It is therefore an object of this invention to provide a single-component powdered resin for use in electrostatic powder-spray coating devices, which would be capable of baking in the shortest possible time and would produce coatings, lacquers or films having improved properties, especially a high gloss.
In the broadest practice contemplated, a hydroxyl containing copolymer is reacted with a mono-blocked diisocyanate while the mixture is in the melt stage, then cooled and pulverised to form a powdered, blocked single-component resin especially useful for electrostatic spray coating applications.
The hydroxyl-containing copolymer used in the practical application of the invention, is formed by copolymerising a vinylaromatic hydrocarbon with an alpha,beta- unsaturated dicarboxylic acid anhydride, subsequently half-esterifying the copolymer with an aliphatic alcohol or a glycol monoether, and alkoxylating the resultant half-esterified copolymer with an alkylene oxide.
In order to further aid in the understanding of the inventive concept, the following additional disclosure is submitted.